Patent Reference No. 1 discloses a method in which (2Z)-4-[(4-aminophenyl)amino]-4-oxo-2-butene acid sodium and carbon black are placed in a Banbury mixer and kneaded with rubber (hereinafter “prior manufacturing method”). Regarding (2Z)-4-[(4-aminophenyl)amino]-4-oxo-2-butene acid sodium, Patent Reference No. 1 further discloses that the terminal nitrogen functional group bonds to carbon black and that the carbon-carbon double-bond portion bonds to polymer.